Modern vehicular emissions control systems typically employ a catalytic converter to reduce hydrocarbon emissions. The catalytic converter contains a catalyst which converts unburned exhaust hydrocarbons to less environmentally detrimental exhaust gases.
Unfortunately, modern catalytic converters only operate after reaching temperatures in excess of about 300 degrees Centigrade. For this reason, a substantial portion of hydrocarbon emissions occur during the first few minutes of cold-start engine operation before the converter reaches its minimum effective operating temperature, otherwise known as the converter "light-off" temperature. Because the first few minutes of operation is an integral part of automotive emissions tests, and because over 60% of the measured hydrocarbons are emitted during the cold-start period of the test, reducing cold-start hydrocarbon emissions is of critical importance. Recent tightening of emissions requirements to limit emissions of certain hydrocarbon compounds such as benzene has further underscored the need for reduced cold-start hydrocarbon emissions.
To reduce cold-start hydrocarbon emissions, emissions control designers have proposed routing exhaust gases through hydrocarbon adsorbers such as charcoal for a short period of time following an engine cold-start. For example, Templin, U.S. Pat. No. 3,645,098 teaches the use of an exhaust gas valve downstream of a catalytic converter for directing unconverted cold-start hydrocarbons onto a charcoal adsorber. As adsorber temperature increases, hydrocarbons initially adsorbed during the cold-start period are released from the adsorber and recirculated into the engine or exhaust manifold. Once the catalytic converter reaches its light-off temperature, the exhaust gas valve routes exhaust gas directly from the catalytic converter to the tailpipe.
While Templin's system might reduce hydrocarbon emissions below the levels emitted from similar systems lacking an adsorber, his system is not preferred because the system requires an exhaust gas valve to operate reliably under the severe chemical and temperature conditions present in the exhaust gas stream and because the physical adsorbance efficiency of his absorber is likely to decrease significantly with increasing exhaust gas temperature.
To overcome the disadvantages of systems like Templin's, other designers have turned to multi-adsorber systems. In these systems, exhaust gas flow is directed first to a low temperature adsorber chamber. As system temperature increases, flow is directed around the low temperature adsorber chamber to a second adsorber chamber containing an adsorber useful in a temperature range above that of the low temperature adsorber and below the catalytic converter light-off temperature. One example of such a system is disclosed in Minami, U.S. Pat. No. 4,985,210.
Minami discloses a system in which cold-start exhaust gas initially flows serially through a charcoal adsorber chamber, a Y-zeolite or mordenite adsorber chamber and a catalytic converter. When exhaust gas temperature reaches a predetermined level, an exhaust gas valve operates to route exhaust gas around the charcoal adsorber and directly into the second adsorption chamber containing the mordenite or zeolite. Because the second adsorber is believed to provide some additional hydrocarbon hold-up at temperatures exceeding the upper useful temperature of the charcoal adsorber, emissions may be reduced from the levels emitted from systems like Templin's. Unfortunately, like Templin's, Minami's system also employs an exhaust gas valve which must function reliably under the harsh physical and chemical conditions found in exhaust gas streams. Additionally, because exhaust gas passes directly into Minami's adsorbers, heat is lost in the adsorbers, thereby delaying catalytic converter light-off.
To avoid the reliability problems inherent in valved emissions systems, other designers have turned to non-valved designs combining an adsorber and a catalytic converter in a single unit. One such example is disclosed in U.S. Pat. No. 3,067,002 to Reid. Reid discloses an exhaust gas emissions control system in which a plurality of catalyst-containing channels are interspersed with a plurality of manifolded open ducts within a housing. As exhaust gas passes through the open ducts, the gas indirectly heats the catalyst contained in the catalyst beds prior to the exhaust gas entering the beds. Reid states that an adsorbent such as a natural or synthetic zeolite can be incorporated into a portion of each catalyst bed.
While Reid's design might reduce the time before catalytic converter light-off, the design appears to preclude the use of heat-damageable adsorbers such as charcoal because exhaust gas must continually pass through the adsorber at all times while the engine is running. More significantly, Reid's physical arrangement of interspersed heat transfer ducts, adsorbent and catalyst within a single envelope appears to limit the potential temperature difference between adsorber and catalyst, thereby limiting the potential effectiveness of his system.
What is needed is a mechanically simple, valveless exhaust gas emissions control system that employs one or more adsorbents to reduce hydrocarbon emissions over at least a substantial portion of the time period between an engine cold-start and catalytic converter light-off.